Interpretive Summary: Winter wheat is sown in the autumn and harvested the following summer, thus the plants must survive subfreezing temperatures for several months. Although freezing tolerance has been studied for several decades, very little is known about the impact on survival of individual components of the freezing process. In this study, the freezing process was described as the cooling rate, the minimum temperature, the time at the minimum temperature, and the warming rate as the freezing episode ended. The impact of each of these components on the survival of 22 wheat cultivars was evaluated. Each component significantly impacted survival of at least some of the cultivars. The 22 cultivars comprised four distinct groups when clustered according to their response to these components. These results showed that the impact of individual components of the freezing process can be determined, and that genetic variation in the ability to tolerate changes in the components exists. This new information provides a completely new way of looking at freezing tolerance, and new traits to target in breeding for improved winter hardiness.

Technical Abstract:
Winter wheat is sown in the autumn and harvested the following summer, necessitating the ability to survive subfreezing temperatures for several months. Because of autumn rains and winter snows, the plants usually are subjected to these subfreezing temperatures while growing in saturated soil. As the plants freeze, they are subjected to freezing episodes that may vary in the cooling rate, the minimum temperature, the time at the minimum temperature, and the warming rate as the freezing episode ends. This study was conducted to investigate the impact of each of these freezing process components on the ability of 22 winter wheat cultivars to survive freezing in saturated soil. Seedlings that had been acclimated at +4°C for 5 weeks in saturated soil were frozen to potentially damaging temperatures under various levels of these freezing process components. Logistic regression analysis and cluster analyses were used to evaluate the impact of each of the freezing process components on survival. The 22 cultivars formed four distinct groups when clustered on the odds ratios associated with each component of the freezing process. The distinctiveness of the four clusters indicated the cultivars in each cluster responded differently to the freezing process components than the cultivars in the other clusters. The levels of freezing tolerance among the cultivars within each cluster generally were similar. However, several examples of cultivars that occurred in different clusters but had equal levels of freezing tolerance also were found. We conclude the logistic regression/clustering analysis identified cultivars that differed in the primary mechanisms used to respond to freezing stress, and that equal levels of freezing tolerance can be attained through these different mechanisms. It may be possible to improve winterhardiness of winter wheat grown in saturated soil by genetically combining these disparate responses to the freezing process components.